Life's timekeeper is a 'free-running' intracellular oscillator synchronised across all cells. It runs throughout life splitting lifespan into equal length phases. During the maturational period it controls the overall rate of progression whereas in the post-maturational period it controls the overall rate of ageing. This includes the rate of senescence and hence time to death. As such life's timekeeper equates maturational and post-maturational time, hence explains the tight correlation between these time periods that has existed throughout mammalian evolution. Life's timekeeper is proposed to have played an important role in vertebrate evolution. A slower oscillatory frequency results in proportional life phase prolongation. This leads to increased body and brain size, together with extended lifespan. Higher brain centres, neocortex in mammals, are disproportionately enlarged. Hence behavioural capacity is increased. The extended post-maturational period ensures that there is enough time in order that the behavioural advantages can be fully manifest in the environment. A faster oscillatory frequency would result in proportional life phase reduction. This process however would lead to reduced behavioural capacity, and is hence unlikely to be positively selected. Therefore throughout evolution life's timekeeper has operated to extend lifespan. It has hence functioned to promote longevity as opposed to ageing.